For some reason, your user name was omitted, but I found it via email address.

If there's no grit on the tile or plastics, you need to take a look underwater with a mask or goggles, and see if it's raised or depressed.

Tell me about the "bubbler" -- what is it, where is it, when does it run . . . and most important, did you maintain a normal (<7.8) pH while it ran?

I'm afraid I may know what happened; telling you my suspicions may help you confirm or deny them:
1. Your 'bubbler' -- presumably some sort of continuous aeration -- ran quite a bit.
2. Because you were a conscientious pool owner, you maintained a proper pH . . . but you didn't check TA all that often.
3. Because the pool industry -- except for my site and a couple of derivative sites -- does not understand how carbonate alkalinity works in pools, no one warned you what happens if you (a) aerate and (b) maintain the pH. This page -- http://pool9.net/alk-step/ -- explains what happens.
4. You ended up running a pool with VERY low carbonate alkalinity, which eroded your plaster, even though the pH was normal.
5. Consequently, you ended up, not with scale, but with corroded plaster.

An inspection from 6" away with goggles will tell. It will be easier if you do so early or late in the day, so the light striking the plaster at an angle, rather than from directly overhead.